A known type of semiconductor device based on nitride semiconductors is a high electron mobility transistor (HEMT) including a laminate of a buffer layer, a GaN (gallium nitride) channel layer, and an AlGaN (aluminum gallium nitride) electron supplying layer. A HEMT operates through the use of carriers of high concentration generated at the interface between a channel layer and an electron supplying layer.
Nitride semiconductors have higher breakdown voltages than GaAs (gallium arsenide) based semiconductors and are thus operable under high voltages. For example, HEMTs formed with nitride semiconductors have power densities ten times higher than HEMTs formed with GaAs semiconductors. Hence, nitride-semiconductor HEMTs are used in devices of high-power and high-frequency performance.
Patent Literature 1 describes a technique to use a double heterostructure of n-type AlGaAs (aluminum gallium arsenide) barrier layer/n-type InGaAs (indium gallium arsenide) channel layer/n-type AlGaAs layer for a HEMT in order to improve its distortion characteristic. Through this technique, the flatness of Gm can be improved. Thus, the distortion characteristic of HEMTs can be improved. Patent Literature 2 describes a double heterostructure for a nitride-semiconductor HEMT.
Patent Literature 1 Japanese Patent Application Laid-Open Publication No. H06-97204
Patent Literature 2: Japanese Patent Application Laid-Open Publication No. 2008-288474